1. Field of the Invention
This invention relates to rear wheel steering devices for use in four wheel steered vehicles (4WS) and, more particularly, to rear wheel steering devices driven by an electric motor.
2. Description of the Related Art
In recent years, 4WS or four-wheel steering has become popular because it improves motion characteristics of a vehicle by steering rear wheels in conjunction with front wheel steering. It is essential for such 4WS to install a rear wheel steering device to steer rear wheels in conjunction with front wheel steering. This type of rear wheel steering device is disclosed in Japanese Patent Laid-Open Publication No. 2000-79881 filed by the applicant of the present application.
The aforementioned rear wheel steering device is shown in FIGS. 13A to 13C hereof. An operation of the aforementioned rear wheel steering device will be described below with reference to those drawings.
In FIG. 13A, a lifting screw 102 is rotated by an electric motor 101, thereby moving left and right slide blocks 103L and 103R to the left and right sides, respectively.
Since the left slide block 103L remains in a clutch off state, the left side block 103L is moved to the left side with the position of left clutch ring 104L remaining unchanged.
The right slide block 103R remains in a clutch-on state, and consequently, a plurality of internal teeth 105R, which inwardly project from an inner periphery of the right slide block 103R, may push a plurality of external teeth 114R which outwardly project from an outer periphery of the right clutch ring 104R. Therefore, sliding forth is sequentially transmitted to the right slide block 103R, to right clutch ring 104R, to right lock ring 106R and then to steering shaft 107, moving a steering shaft 107 to the right direction as shown by arrow to steer rear wheels.
In such a condition, left and right arms 110L and 110R, which project outwardly from outer peripheries of the left and right clutch rings 104L and 104R, respectively, and which engage in a guide slot 109 (shown in the upper portion of FIG. 13A) formed in an inner periphery of a case (not shown), are movable to the right direction, so that they do not interrupt an axial movement of the steering shaft 107.
An incremental stroke value relative to the center of the steering shaft 107 can be adjusted without any restraint by rotating the electric motor 101, and can be measured by a steering angle sensor 112. Since the steering angle corresponds to a deflected angle of the wheel (designated by reference character xcex8 in FIG. 14) and the steering angle is obtained by converting the incremental stroke value into an angle, a stroke sensor will be referred to as a steering angle sensor 112.
The incremental stroke value of the left slide block 103L can be measured with a driving angle sensor 113. Since the driving angle is determined by converting the incremental stroke of the slide block 103L into an angle, a stroke sensor will be referred to as a driving angle sensor 113.
FIG. 13B shows that the right clutch ring 104R has been moved to the right direction by the internal teeth 105R in abutting engagement with the external teeth 114R.
FIG. 13C shows a relationship between the right slide block 103R, after the electric motor 101 in FIG. 13A has been rotated in a reversed direction, and the central flange 115 that radially protrudes in a central area of an axial direction of the steering shaft 107. By rotating the lifting screw 102 in a reverse direction, the right slide block 103R is moved leftward to cause the right slide block 103R to directly push the central flange 115 leftward.
FIG. 14 shows an operational principle of the conventional rear wheel steering device. A control device 117 receives the steering angle control data and starts the electric motor 101 responsive to the control data. When the lifting screw 102 is rotated by the electric motor 101, the left and right slide blocks 103L and 103R are moved away from each other. The steering shaft 107 is moved to the right direction by the right slide block 103R. The control device 117 compares the steering angle data delivered from the steering angle sensor 112 with the steering angle control data (produced by the front wheel steering system, not shown)and controls the steering angle xcex8 based on the steering angle control data such that, when the steering angles xcex8 of the rear wheels 118L and 118R coincide with the steering angle control data, the operation of the electric motor 101 is stopped.
The electric motor 101 must generate output power sufficient enough to steer the rear wheels to a specified direction immediately. The more output power is required, the larger the external diametrical dimensions (external dimension and length) of the electric motor become. Larger external diametrical dimensions interrupt miniaturization of the rear wheel steering device. Generally, small-sized electric motors can be mass produced and hence are inexpensive. On the other hand, production volume of large-sized electric motors is too small to be cost effective. The electric motor 101 has a large rated output power and hence is expensive. This prevents cost reduction of the rear wheel steering device.
It is therefore an object of the present invention to provide a rear wheel steering device which is compact in construction and low in manufacturing cost by rendering electric motors compact in size and low in cost.
According to an aspect of the present invention, there is provided a rear wheel steering device for use in a vehicle, which device comprises two electric motors serving as drive sources for steering rear wheels of the vehicle, and a rear wheel steering control device for controlling the two electric motors to drive in case both of the two electric motors are judged to operate in a normal condition on the basis of motor condition signals indicative of how the two electric motors are working.
The rear wheel steering device is driven by two electric motors. If output power is reduced by 50%, it becomes possible to employ small sized and low cost electric motors This results in achieving the desired miniaturization and cost reduction of the rear wheel steering device.
Desirably, the rear wheel steering device further includes a vehicle speed sensor for detecting a vehicle speed. The rear wheel steering device may be designed such that when one of the electric motors is in an abnormal state while the other one of the electric motors is in a normal state and when the vehicle speed detected by the vehicle speed sensor exceeds a give speed, it controls steering of the rear wheels using only the one electric motor being in normal state. That is, even when one of the electric motors goes out of a normal condition, the rear wheels can be steered using only the other electric motor remaining in normal condition as long as a given condition is satisfied. The given condition may be when the vehicle speed exceeds a predetermined speed. When the vehicle speed is below the predetermined value, the resulting steering torque is large and this will act as an overload on the electric motor of xc2xd (50%) output power. Thus, the rear wheels are steered only by the normal electric motor when the vehicle speed exceeds the given vehicle speed.
Preferably, the rear wheel steering device further comprises a vehicle speed sensor for detecting a vehicle speed. When one of the electric motors is in an abnormal state while the other one of the electric motors is in a normal state and when the vehicle speed detected by the vehicle speed sensor exceeds a given speed, the rear wheel steering control device controls, using only the in-normal-state electric motor, steering of the rear wheels to cause the rear wheels to be returned to a neutral position. That is, when a certain condition is satisfied, the rear wheels can be returned to the neutral position using only the normal electric motor even though defects are detected in either one of the two electric motors. The certain condition may be when the vehicle speed exceeds the given vehicle speed. When the vehicle speed is over the given speed, small steering torque is required for steering the rear wheels and hence only the normal electric motor of xc2xd power output can achieve the intended drive control. Accordingly, the rear wheels are arranged to be steered using only the normal electric motor to cause the rear wheels to be returned to the neutral position when the vehicle speed exceeds the given vehicle speed.